Sorry to hear about the snail... Nice thing about it is you usually don't have a hard time finding more, if you wish to replace them. Apple snails almost seem to be a pest to some tanks due to infestation. Just got some Amazon Frogbit today and found seven baby snails amongst the leaves, and countless eggs. And honestly, I doubt you killed him... if he was really slimy, there was a cloud, and he didn't pull back in his shell, I can't imagine he was alive by that point.
As for the water... hard water is really rough on fish to begin with. So if you have pretty hard water to begin with, you're not going to have healthy fish. This would probably explain why the fish looked sick and would gulp for air or be motionless.
Most tropical fish I've read about need soft water, though there are some that need hard--best to do your research if you have very hard or very soft water. According to the Wiki, take it for what it's worth, guppies actually prefer a hard water aquarium. Though given their hardiness and ability to exist in saltwater as well, they probably can thrive in soft water too. They also prefer slightly basic water, so that's another thing to keep in mind.
I wouldn't really let all that get to you though. If your guppies are actually swimming around and not displaying the same symptoms as your last number of attempts, why fix what's not broken? I just thought you should know, especially if you wish to expand your tank or get different types of fish.